基于环境激励的大跨结构动力特性识别

本文以广州国际会展中心一期在环境激励下获得的屋盖5处位置加速度响应时程数据为基础,分别采用随机减量法(RDT)、峰值拾取法(PP)以及频域分解法(FDD)识别结构的前4阶竖向振动模态频率;并采用随机减量法(RDT)获得了前2阶阻尼比。基于有限元分析软件ANSYS,建立结构三维有限元模型,获得了结构前16阶模态频率,并将前4阶整体竖向模态频率与实测结果进行对比。研究表明:(1)RDT法、PP法以及FDD法具有很好的识别结构振动模态频率的功能,且RDT法能有效地识别结构整体阻尼比;(2)有限元数值计算在结构设计初期对识别动力特性上具有较高的精度。     

环境激励下ERA方法在砌体结构振动测试中的应用研究

ERA方法是基于环境激励的结构振动测试的方法中重要的时域分析方法。主要由ERA算法对齐齐哈尔砌体结构居民房的基本模态参数进行测试。简述了ERA算法的主要思路和计算过程,介绍了相应的模态识别准则MAC,以及ERA算法在MATLAB中的实现。由ERA算法得到的模态参数与有限元建模分析结果分析比较吻合,为砌体结构在环境激励下用ERA方法测试模态参数提供了实验依据。最后,讨论了ERA方法与有限元建模分析结果出现差异的原因,以及ERA方法在环境激励下的限制和不足。     

银川海宝塔动力特性测试及抗震性能分析

为了给银川市海宝塔的修缮保护工作提供参考依据,对其动力特性及结构损伤状况开展测试和数值模拟分析。首先对海宝塔的材料强度进行测试和计算,得出塔砌体的抗压强度及弹性模量;然后对该塔进行环境激励作用下的振动响应测试,得到塔东西和南北方向的1阶频率;最后建立数值模型并分析其模态与抗震性能,得到海宝塔的动力特性与地震作用下塔体各层的层间位移角,并基于砖石古塔破坏状态位移准则以及损伤指标开展损伤分析。结果表明：海宝塔东西方向和南北方向的1阶频率较为接近,但不同楼层的振动频谱曲线差异较大,尤其是峰值点数量;楼层越高,最大位移和层间位移角越大,受损也越严重;结构总体刚度退化较为严重,存在较大安全隐患,需进一步开展深入分析并采取保护措施。     

基于经验模式分解的随机子空间识别方法

提出了基于经验模式分解（EMD）的环境激励结构模态参数随机子空间识别（SSI）方法。该方法用设置间断频率的EMD将结构环境振动响应原始信号分解成若干个基本模式分量（IMF）,使每一个基本模式分量仅为结构的某一阶固有模态,进而用随机子空间方法进行模态参数识别。实桥环境振动实验分析结果表明,该方法能有效地避免结构各阶模态之间的相互影响,能够更清晰方便地得到结构的模态参数。     

基于损伤部位向量法的钢框架损伤识别研究

对损伤部位向量(DLV)法作了简单介绍,并用该方法对钢框架进行了损伤识别和损伤定位。该方法假定结构损伤前后为线性,对结构损伤前后柔度矩阵差进行奇异值分解,将奇异值为零所对应的向量,作为静荷载施加在无损结构的测点位置,则应力为零的单元为可能损伤的单元。对3种不同工况的钢框架进行了振动模态试验,用前3阶模态参数构造框架的柔度矩阵,按照DLV法对其进行了损伤识别,识别结果与已知损伤情况相一致。从测试自由度不完备、噪声和振型质量归一化系数这3个方面对识别效果进行了分析,结果表明:当损伤使结构动力特性有微小改变时,使用该方法不易定位损伤,应结合局部损伤识别方法进行判定;当损伤使结构动力特性有较大改变时,该方法能有效识别损伤的单元。DLV方法概念简单,理论明确,不受结构类型的限制,不需要结构的数学模型和模型缩聚或扩展技术,只需获得结构损伤前后的前几个低阶模态参数,即可识别结构一处或多处损伤,实际应用时可操作性强。     

基于两阶段稳定图的随机子空间识别结构模态参数

基于振动的结构健康监测的前提是从振动测试信号中提取结构模态参数。随机子空间方法是近年来发展起来的一种线性系统辨识方法,可以有效地从环境激励的结构响应信号中提取结构模态参数。在随机子空间识别方法中,确定系统的阶数是该方法的关键工作,稳定图方法是一种比较新颖的确定系统阶次的方法。但是随机子空间方法容易产生虚假模态,这也是随机子空间方法的一个主要缺陷。因此针对于这一缺陷提出了一种基于两阶段稳定图的随机子空间识别结构模态参数方法,该方法的基本思想是将在现场采集的结构的输出信号进行分段,将各段信号用随机子空间结合稳定图进行识别,然后将所有各段所识别的模态参数再一次用稳定图方法进行分析,得出结构的模态参数。最后用一三跨连续梁的数值模型对该方法进行验证,结果表明该方法具有良好的识别效果。     

某首层柱顶隔震结构动力特性测试研究

为了达到隔震设计的目的并检验隔震层的工作性能,测试分析了采用首层柱顶隔震技术的某五层框架结构在环境激励和初位移条件下的动力特性,得到了结构的前3阶自振特性如自振频率、振型和阻尼比;分析了初位移条件下隔震层的自由衰减曲线和滞回性能.结果表明:环境激励条件下,隔震结构与非隔震结构动力特性相近,隔震结构在正常使用状态下具有足...     

基于Hilbert-Huang变换和随机子空间识别的模态参数识别

基于Hilbert-Huang变换和随机子空间识别技术提出了两种土木工程结构的模态参数识别方法。方法一是基于Hilbert-Huang变换和自然激励技术,通过经验模态分解和Hilbert变换提取信号的瞬时特性,进而利用自然激励技术和模态分析的基本理论识别结构的模态参数;方法二是基于经验模态分解和随机子空间识别技术,通过经验模态分解对信号进行预处理,进而运用随机子空间识别方法处理得到的结构单阶模态响应以识别结构的模态参数。利用这两种方法,通过对一12层钢筋混凝土框架模型振动台试验测点加速度记录的处理,识别了该模型结构的模态参数。识别结果与传统的基于傅里叶变换的识别结果及有限元分析结果的对比验证了这两种方法的可行性和实用性。     

无填充墙基础隔震RC框架在环境激励下的动力特性

对一基础隔震钢筋混凝土框架结构在无填充墙情况下进行了环境激励下的动力测试,重点利用Hilbert-Huang变换与随机减量技术相结合的方法识别了其模态参数,并与随机子空间识别法、有理分式多项式法识别的结果进行了对比。识别结果表明在环境激励下,基础隔震结构的基本周期远小于多遇和罕遇地震工况下设计计算的基本周期;等效黏滞阻尼比很小,近乎于基础固定模型。对隔震层阻尼特性的分析表明,环境激励下可以将基础隔震结构视为经典的比例阻尼系统。进一步以识别的模态参数为基准,采用优化的方法数值反演了环境激励下该结构隔震层的实际水平等效刚度,结果表明其值为多遇地震下计算刚度取值的10.75倍。     

在线监测环境下土木结构的模态识别研究

建立在线监测环境下结构考虑时效的模态识别计算方案。采用基于各测点加速度响应互功率谱的频域多参考点模态识别法来实现结构模态参数的抽取,从而绕过了监测环境下激励监测的技术难题;并用频域的平均法使识别参数的拟合曲线平滑和发现参数的变化趋势。通过对美国结构健康监测研究小组公布的Benchmark问题的第一阶段解析模型模拟加速度响应数据的识别,表明本文采用的算法有较好的识别精度和识别速度,是一个可行的在线监测环境下的模态识别计算方案。     

应县木塔环境振动试验

对于复杂的塔状古建筑物,建立一个合适的简化的数学模型是不易实现的。与强迫振动试验相比,环境振动试验比较简单、方便,所确定出的小振幅水平下的木塔结构动力特性具有足够的精度。本文首先结合应县木塔结构环境振动测试实例对环境振动测试方法作了简要介绍,并采用随机信号数据频域分析方法对测试数据进行了分析,确定木塔结构的自振频率。同时,依据不同测点在固有频率处响应的比及零迟时互相关函数确定木塔结构的振型。最后,本文依据自功率谱和互功率谱采用半功率点法计算各振型的阻尼比。本文分析结果表明,在两水平方向上木塔的振动特性存在些微差异,第一振型阻尼比较第二振型阻尼比大。     

Dynamics of towers surrounded by water

The effects of interaction with surrounding water on the dynamic response behaviour of cantilever tower structures are studied. Expressions for response to harmonic ground motion in individual modes of vibration, including hydrodynamic interaction, are presented, the accuracy of responses obtained by ignoring surface waves and compressibility of water in the hydrodynamic solutions is evaluated, the effects of hydrodynamic interaction on the fundamental period of vibration are studied and the commonly used ‘added mass’ approach to account for effects of surrounding water is examined. The conclusions deduced from the results of this investigation include the following. Interaction with surrounding water increases the fundamental period of vibration of the tower and decreases the modal damping ratio. Compressibility of water has essentially no influence in the hydrodynamic effects on slender towers. The traditional definition of added mass is conceptually deficient, but is simple to employ; the errors in this simple added mass representation are negligible for the first mode of vibration of towers.     

Ambient vibration measurements on a cable-stayed bridge

An extensive programme of full-scale ambient vibration tests has been conducted to measure the dynamic response of a 542 m (centre span of 274 m) cable-stayed bridge—the Quincy Bayview Bridge in Illinois. A microcomputer-based system was used to collect and analyse the ambient vibration data. A total of 25 modal frequencies and associated mode shapes were identified for the deck structure within the frequency range of 0–2 Hz. Also, estimations were made for damping ratios. The experimental data clearly indicated the occurrence of many closely spaced modal frequencies and spatially complicated mode shapes. Most tower modes were found to be associated with the deck modes, implying a considerable interaction between the deck and tower structure. No detectable levels of motion were evident at the foundation support of the pier. The results of the ambient vibration survey were compared to modal frequencies and mode shapes computed using a three-dimensional finite element model of the bridge. For most modes, the analytic and experimental modal frequencies and mode shapes compare quite well, especially for the vertical modes. Based on the findings of this study, a linear elastic finite element model appears to be capable of capturing much of the complex dynamic behaviour of the bridge with very good accuracy, when compared to the low-level dynamic responses induced by ambient wind and traffic excitations.     

Full-scale dynamic testing and analysis of a reservoir intake tower

Dynamic tests were conducted on a 50 m high intake tower at Wimbleball dam in the U.K. The results were compared against predictions from a corresponding numerical model. The aim of this work was to validate the assumption that the compressibility of the reservoir water is not a significant factor in the seismic analysis of intake towers. Three sets of tests were conducted on different occasions with different water levels in the reservoir. In the first two tests, modal characteristics of the tower were determined from the measured responses under ambient, hammer and human excitation. These results were used in planning the final set of tests where rotating eccentric mass exciters were used to vibrate the tower. Structural accelerations and hydrodynamic pressures were measured over the height of the tower for three important bending modes of vibration. The finite element method was used to develop a numerical model for Wimbleball tower. The tower was discretized with traditional solid elements and the reservoir with incompressible fluid elements. This model was analysed to predict the modal characteristics and harmonic responses of the tower and reservoir under the various conditions imposed during the dynamic tests. Theoretical predictions of the tower's accelerations and hydrodynamic pressures in the reservoir were compared against the test results. Excellent agreement was found for the natural frequencies and mode shapes while predictions of the harmonic responses were only fair. The observed responses of the tower and reservoir support the assumption that reservoir compressibility is not a significant factor in the seismic analysis of towers of this size.     

基于土弹簧的桩-土-输电塔抗震性能研究

在传统输电塔抗震分析中,通常假设输电塔固定在地面上,而不考虑土G结构相互作用(SoilG Structure Interaction,SSI)对结构的影响,从而可能导致输电塔抗震性能评估不准确.依托某 1000kV输电铁塔实际工程,对地震动激励下考虑SSI的输电塔进行综合的地震响应、倒塌破坏和倒塌易损性分析.在 ABAQUS中,通过沿桩体设置依据 API规范确定的零长度弹簧模拟土体与桩体间的相互作用,建立考虑SSI的输电塔有限元模型.基于数值模型,开展输电塔动力特性和结构动力响应分析.采用增量动力分析(Incremental Dynamic Analysis,IDA)方法,研究SSI对输电塔倒塌机理和易损性的影响.结果表明:考虑和未考虑SSI的输电塔振型相似,但自振频率相差较大;忽略SSI会低估输电塔的地震响应,同时高估输电塔的抗倒塌能力.因此考虑SSI可以提高输电塔抗震能力,进而确保输电线路的安全性.     

Rapid seismic analysis methodology for in-service wind turbine towers

The wind energy industry has been growing rapidly during the past decades.Along with this growth,engineering problems have gradually emerged in the wind power industry,including those related to the structural reliability of turbine towers.This study proposes a rapid seismic analysis methodology for existing wind turbine tower structures.The method is demonstrated and validated using a case study on a 1.5 MW tubular steel wind turbine tower.Three finite element(FE)models are developed first.Field tests are conducted to obtain the turbine tower's vibrational characteristics.The tests include(1) remotely measuring the tower vibration frequencies using a long range laser Doppler Vibrometer and(2) monitoring the tower structural vibration by mounting accelerometers along the height of the tubular structure.In-situ measurements are used to validate and update the FE models of the wind turbine tower.With the updated FE model that represents the practical structural conditions,seismic analyses are performed to study the structural failure,which is defined by the steel yielding of the tubular tower.This research is anticipated to benefit the management of the increasing number of wind energy converters by providing an understanding of the seismic assessment of existing tubular steel wind turbine towers.     

Earthquake response of steel-framed multistorey buildings with set-backs

A study is made of the dynamic behaviour of multistorey steel rigid-frame buildings with set-back towers. The effects of set-backs upon the building frequencies and mode shapes are examined. Then the effects of set-backs on seismic response are investigated by analysing the response of a series of set-back building frame models to the El Centro ground motion. Finally, the computed responses to the El Centro earthquake are compared with some code provisions dealing with the seismic design of set-back buildings. The conclusions derived from the study include the following:

• 1. The higher modes of vibration of a set-back building can make a very substantial contribution to its total seismic response; this contribution increases with the slenderness of the tower.
• 2. Some of the important response parameters for the tower portion of a set-back building are substantially larger than for a related uniform building.
• 3. For very slender towers, the transition region between the tower and the base may be subjected to very large storey shears.

     

Ambient vibration survey of the fatih sultan mehmet (second Bosporus) suspension bridge

Ambient accelerations due to dynamic excitation by wind and traffic were measured on the deck, towers, cables and hangers of the Fatih suspension bridge. From these measurements it was possible to obtain natural frequencies, mode shapes and damping ratios for vertical, lateral, torsional and associated modes in the deck and tower up to a maximum of 2 Hz. The objective of the test was to validate the mathematical modelling used in seismic analyses of the bridge. The agreement between the experimental and theoretical modes was acceptable for vertical modes below 1 Hz, and for torsional modes, but it was difficult to identify the lateral modes due to low levels of response. The dynamic behaviour of this bridge and two other major European suspension bridges is discussed in relation to the differences in loading and structural design.     

Earthquake analysis of axisymmetric towers partially submerged in water

A method for analysis of response of axisymmetric towers partly submerged in water to earthquake ground motion is presented. The tower is idealized as a finite element system. The hydrodynamic terms are determined by solving the Laplace equation, governing the dynamics of incompressible fluids, subject to appropriate boundary conditions. For cylindrical towers, these solutions are obtained as explicit mathematical solutions of the boundary value problems; whereas they are obtained by the finite element method in case of towers with non-cylindrical outside surface. The response to earthquake ground motion is determined by step-by-step integration of the equations of motion. Analyses of two actual intake towers are presented to illustrate results obtained by this method. The small computation times required for these analyses demonstrate that the method is very efficient. The effectiveness of this formulation lies in avoiding the analysis of a large system by using a substructure approach and in exploiting the important feature that structural response to earthquake ground motion is essentially contained in the first few modes of vibration of the tower with no surrounding water.     

Hilbert-Huang变换在密频结构阻尼识别中的应用

Hilbert—Huang变换是一种新的数据处理方法，由经验模分解(Empirical Mode Decomposition)技术及Hilbert变换两部分组成。本文研究此方法对于密频结构阻尼识别的应用。首先对于两自由度系统模型，说明该方法用于阻尼识别的步骤。进而研究存在频率密集现象的高层建筑的阻尼识别问题。上述结果与理论值及由半功率带宽法的识别值进行了比较，对比显示Hilbert．Huang方法较传统方法具有良好的识别密频结构阻尼的性能，适用于大型结构的系统识别。